Forming sacrificial composite materials for package-on-package architectures and structures formed thereby

ABSTRACT

Methods of forming a microelectronic packaging structure are described. Those methods may include forming a solder paste comprising a sacrificial polymer on a substrate, curing the solder paste below a reflow temperature of the solder to form a solid composite hybrid bump on the conductive pads, forming a molding compound around the solid composite hybrid bump, and reflowing the hybrid bump, wherein the sacrificial polymer is substantially decomposed.

BACKGROUND OF THE INVENTION

As semiconductor technology advances for higher processor performance,advances in packaging architectures may include package-on-package (POP)architecture and other assemblies that may require exposed die-sidesolder bumps (for enabling of ball on ball (BoB) technologies, forexample). Such packaging architectures enable solder joint formationwith a top package.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming certain embodiments of the present invention,the advantages of this invention can be more readily ascertained fromthe following description of the invention when read in conjunction withthe accompanying drawings in which:

FIGS. 1 a-1 e represent methods of forming structures according to anembodiment of the present invention.

FIGS. 2 a-2 e represents methods of forming structures according toanother embodiment of the present invention.

FIGS. 3 a-3 b represents structures according to an embodiment of thepresent invention.

FIG. 3 c represents a structure according to the Prior Art.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following detailed description, reference is made to theaccompanying drawings that show, by way of illustration, specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention. It is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described herein, in connection with one embodiment, maybe implemented within other embodiments without departing from thespirit and scope of the invention. In addition, it is to be understoodthat the location or arrangement of individual elements within eachdisclosed embodiment may be modified without departing from the spiritand scope of the invention. The following detailed description is,therefore, not to be taken in a limiting sense, and the scope of thepresent invention is defined only by the appended claims, appropriatelyinterpreted, along with the full range of equivalents to which theclaims are entitled. In the drawings, like numerals refer to the same orsimilar functionality throughout the several views.

Methods and associated structures of forming and utilizing amicroelectronic structure, such as a solder interconnect structure, aredescribed. Those methods may comprise forming a solder paste comprisinga sacrificial polymer on a substrate, curing the solder paste below areflow temperature of the solder to form a solid composite hybrid bumpon the conductive pads, forming a molding compound around the solidcomposite hybrid bump, and reflowing the hybrid bump, wherein thethermally decomposable polymer is substantially decomposed. Methods ofthe present invention enable the fabrication of moldedpackage-on-package architectures such as PoP assemblies and otherassemblies that require exposed die-side solder bumps to enable solderjoint formation with a top package.

FIGS. 1 a-1 e illustrate embodiments of a method of forming amicroelectronic structure, such as an interconnect structure, forexample. FIG. 1 a illustrates a substrate 100. In one embodiment, thesubstrate 100 may comprise at least one of a package substrate, amotherboard, an interposer, a test coupon, and a land grid array. Thesubstrate 100 may comprise a portion of an exposed top ball over mold(ET-BOM) or an (exposed side mold) ESM packaging configuration, in someembodiments. The substrate 100 may comprise die side lands/conductivepads 101, in an embodiment. A solder paste 102 comprising a sacrificialpolymer 103 may be formed on the substrate 100 (FIG. 1 b). In anotherembodiment, the solder paste 102 comprising the sacrificial polymer 103may be formed on a land side of the substrate 100. The solder paste 102comprising the sacrificial polymer 103 may be formed on the substrate100 as a plurality of hybrid bump structures 104.

The sacrificial polymer 103 may comprise a thermally decomposablepolymer. In an embodiment, the sacrificial polymer may comprise a classof polymeric materials that may thermally decompose into very lightmolecules, leaving little to no residue behind. In an embodiment, thesacrificial polymer 103 may comprise such materials as polynorbornenesand certain polycarbonates, for example. The sacrificial polymer 103 maycomprise a relatively low decomposition temperature, and in someembodiments, the decomposition temperature may be tunable for a givenapplication.

In an embodiment, the sacrificial polymer may comprise a material withspecific rheological properties (e.g. thixotropy) and decompositiontemperature designed for the particular application. In an embodiment,polycarbonates may comprise a temperature range of between about(150-225 C), and may be applicable in various embodiments. In somecases, no-clean paste/flux materials may be used in electronic packagingstructures utilizing the sacrificial polymer 103 so that minimal residuemay be left behind during subsequent processing, such as during postchip attach processing.

The sacrificial polymer 103 may be evenly dispersed within the solderpaste 102, in one embodiment. The sacrificial polymer 103—solder paste102 composite may be printed on the die side lands 101 of the substratein an embodiment, and/or may be formed onto solder prior to a moldingprocess to be described subsequently herein. In an embodiment, thesolder paste 102 comprising the sacrificial material 103 may bedispensed as a paste/liquid.

In an embodiment, the sacrificial polymer 103 within the hybrid bumpstructures 104 may be cured 106 using heat and/or UV to form a solidcomposite (FIG. 1 c). In some cases the sacrificial polymer 103 of thehybrid bump 104 may not need to be cured. The cure temperature of thesacrificial polymer 102 may be below a reflow temperature of the solderpaste 102, in an embodiment. In an embodiment, the curing of thesacrificial polymer 103 in the solder paste 102 may form a solidcomposite on the conductive pads 101 of the substrate 100. At this pointthe solder paste 102 may not be reflowed, nor is the sacrificialmaterial 103 decomposed.

In this manner, hybrid bump structures 104 with near vertical sidewalls107 can be formed. In an embodiment, the solder paste 102 andsacrificial polymer 103 can be designed to phase separate, such that thelower density sacrificial material 103 may rise to a surface 109 of thehybrid bump structures 104, or alternatively, the solderpaste/sacrificial polymer hybrid bump structure 104 can be designed tomaintain good dispersion of the sacrificial polymer 103 throughout thehybrid bump structure 104.

Next, a molding compound 108 may be formed around the hybrid bumpstructures 104 on the surrounding substrate 100 (FIG. 1 d). In anembodiment, the molding compound may be formed by using either transferor compression molding processes. In an embodiment, the hybrid bumpstructures 104 can be left exposed, wherein the hybrid bump structures104 may comprise a height higher than that of the molding height. Inanother embodiment, the molding compound 108 can be formed over thehybrid bump structures, and a post-mold grinding process (not shown) maythen expose the hybrid bump structures 104. In another embodiment, themolding compound used in such processes as ESM or ET-BOM packages, forexample, may be eliminated, in cases wherein the solder/polymercomposite material has the necessary elasticity to temporarily(elastically) deform during clamping of the mold chase and whereby thehybrid bumps may “self gasket”.

Once the hybrid bump structures 104 are exposed, the sacrificial polymer103 may be burned off from the hybrid bump structure 104 to leave behindthe solder paste on the conductive pads 101 (FIG. 1 e). In anembodiment, the sacrificial polymer may be burned off during a solderreflow process 110, for example. During thermal decomposition of thesacrificial polymer, any mold flash that may have been present on thehybrid bumps 104 may be burned off. The solder paste 102 may be reflowedduring the reflow process 110 to form a plurality of solder interconnectstructures 112/solder bumps disposed on the conductive pads 101 (FIG. 1e).

An opening 114 may be disposed between the molding compound andindividual solder interconnect structure 112. The solder interconnectstructures 112 may comprise a height 116 that is about the same or lowerthan a height 115 of the molding compound 108 in some cases. In anembodiment, further processing may be performed such as but not limitedto a wet etch and/or a plasma etch to improve the shape of the openings114 in the mold compound 108, which may be useful for POP assemblyoptimization. Thus, separation between solder interconnect structures112 and the molding compound 108 may be enhanced, and decoupling betweenthe volume of the opening 114 and the volume of the solder may beachieved.

In an alternative embodiment, solder paste and sacrificial polymer maybe applied sequentially, whereby a solder paste may be applied toconductive pads 201 disposed on a substrate 200 initially to form solderbumps 202 (FIGS. 2 a-2 b). The solder bumps 202 may be formed by using astencil printing and reflow process, for example. A sacrificial polymer203 (which may or may not be subsequently cured) may then be appliedover the solder bumps 202 (FIG. 2 c) just prior to forming a moldingcompound 208 on the substrate 200 surrounding the solder bumps 202 (FIG.2 d). The molding compound 208 may be formed by utilizing a jet dispenseprocess, for example.

In an embodiment, the sacrificial polymer 203 may be burned off during asolder reflow process 210, for example. During thermal decomposition ofthe sacrificial polymer, any mold flash that may have been present onthe solder bumps 202 may be burned off. An opening 214 may be disposedbetween the molding compound 208 and the solder bumps 202, and thesolder bumps 202 may comprise a height that is lower than a height ofthe molding compound 208. This layered structure may achieve many of thesame benefits achieved with the composite paste (e.g. separation betweensolder 202 and mold 208, and decoupling between the volume of theopening 214 and the volume of the solder).

In another embodiment, a portion of a POP structure 300, which maycomprise a portion of a thin die 310 exposure top ball over mold (ETBOM)package structure 300, is shown (FIG. 3 a). The ETBOM package 300 maycomprise solder interconnect structures 302 disposed on a bottom packageportion 301 according to embodiments of the present invention. Greaterprocess control of a side wall angle 311 between a mold compound 308 (ondie side, for example) and the reflowed solder ball 302 may be enabledfor reduced stress concentration in solder joints, such as a low heightsolder joint 303 (FIG. 3 b) that may be formed between the solderinterconnect structures 302 of the bottom package 301 and a solderinterconnect structure 309 of a top package 312 of the POP 300.Additionally the embodiments of the present invention may provide forgreater control of volume shrinkage between solder joints and betweensolder and mold compound/underfill materials. A lower stand off heightis also possible for the PoP package 300 by having a lower height solderjoint, which cannot be achieved with the standard BoB approach. In anembodiment, an opening 313 between the solder ball 302 and the moldcompound 308 is capable of allowing for aligning of the low heightsolder joint 303 in an x-y direction.

Since the solder interconnects 302 in the PoP 300 may be formed by usingsacrificial polymer-solder composite paste (which comprise a lowerheight than prior art solder balls), a Z height 315 of the POP 300 maybe lower than in prior art POP package structures. For example, theprior art package structure 320 shown in FIG. 3 c comprises a standardBOB joint structure. Since the solder balls 322, 324 may comprise ataller height when joined than the solder interconnects of the packagestructure 300, a Z height for the prior art package 320 may be tallerthan a Z height for packages utilizing the solder interconnectstructures according to the embodiments of the present invention, as inthe package structure 300 of FIG. 3 a, for example.

Benefits of the present invention enable the fabrication of sacrificialpolymer-solder composite materials for molded package-on-packagearchitectures. The control of side wall angle for reduced stressconcentration in solder joints is possible, since the top solder in theBoB process may have more space in the opening generated by thesacrificial polymer to accommodate itself. Control of volume shrinkagebetween the solder joints will result in it losing contact with the moldcompound sidewalls, thus creating an escape path for moisture duringsolder reflow (thereby preventing solder extrusion).

The various embodiments provide for the reduction or elimination of moldflash or filler on the solder structures as compared to standard ETBOMand ESM processes. This enables the elimination of a post mold cleaningstep in some case. The pocket-like structure with low solder onconductive pads can potentially enhance alignment integrity of the toppackage for PoP architecture as the side wall of the pocket structuremay prohibit the movement of the structure in the x-y direction.Additionally, the embodiments of the invention can be used on the landside of the package to form the second level interconnect (BGA/SGAequivalent).

Although the foregoing description has specified certain steps andmaterials that may be used in the method of the present invention, thoseskilled in the art will appreciate that many modifications andsubstitutions may be made. Accordingly, it is intended that all suchmodifications, alterations, substitutions and additions be considered tofall within the spirit and scope of the invention as defined by theappended claims. In addition, it is appreciated that variousmicroelectronic structures, such as package structures, are well knownin the art. Therefore, the Figures provided herein illustrate onlyportions of an exemplary microelectronic device that pertains to thepractice of the present invention. Thus the present invention is notlimited to the structures described herein.

What is claimed is:
 1. A method comprising; forming a solder pastecomprising a sacrificial polymer on a substrate; curing the solder pastebelow a reflow temperature of the solder to form a solid compositehybrid bump on the conductive pads; forming a molding compound aroundthe solid composite hybrid bump; and reflowing the hybrid bump, whereinthe sacrificial polymer is substantially decomposed.
 2. The method ofclaim 1 further comprising wherein the substrate comprises one of anETBOM and an ESM substrate configuration.
 3. The method of claim 1further comprising wherein the sacrificial polymer comprises a thermallydecomposable polymer.
 4. The method of claim 3 further comprisingwherein the thermally decomposable polymer leaves little to no residueon the substrate.
 5. The method of claim 1 further comprising whereinmold flash present on the solid composite hybrid bump may be removed. 6.The method of claim 1 further comprising wherein the reflow process ofthe solid composite hybrid bump removes the sacrificial polymer alone.7. The method of claim 1 further comprising wherein the molding compoundcovers the solder paste, and wherein the molding compound is exposed toa grinding process to expose the solder paste.
 8. The method of claim 1further comprising wherein the solid composite hybrid bump is reflown toform a solder bump.
 9. A method comprising; forming a solder bump on adie side land of a package substrate; forming a sacrificial polymer onthe solder bump to form a solder sacrificial polymer composite; curingthe solder sacrificial polymer composite below a reflow temperature ofthe solder bump; forming a molding compound around the soldersacrificial polymer composite to encapsulate the die side of the packagesubstrate; and removing the sacrificial polymer alone by performing areflow process on the solder sacrificial polymer composite to forming asolder interconnect structure.
 10. The method of claim 9 furthercomprising wherein the molding comprises one of an exposed die and anover molding process.
 11. The method of claim 9 further comprisingwherein an opening is formed between the solder interconnect structureand the molding.
 12. The method of claim 11 further comprising shapingthe opening by performing at least one of a wet etch and a plasma etch.13. The method of claim 9 further comprising wherein the sacrificialpolymer comprises a thermally decomposable polymer that decomposes intovery light molecules without leaving any residue behind.
 14. The methodof claim 12 wherein a side wall may angle may be shaped between thesolder interconnect structure and the molding compound.
 15. The methodof claim 9 further comprising wherein the substrate comprises a portionof a low Z height POP package.
 16. A structure comprising: a pluralityof solder interconnect structures disposed onto conductive pads of apackage substrate; and a molding compound disposed around the pluralityof solder interconnect structures, wherein an opening is disposedbetween individual solder interconnect structures and the molding. 17.The structure of claim 16 wherein the package substrate comprises aportion of a POP package structure.
 18. The structure of claim 16wherein the solder interconnect structures comprise low height solderinterconnect structures.
 19. The structure of claim 16 wherein theopening comprises a side wall angle between the molding compound and anindividual solder interconnect structure.
 20. The structure of claim 16wherein the package substrate comprises at least one of a portion of anETBOM or an ESM packaging structure.
 21. The structure of claim 16wherein the solder interconnect structures comprise near verticalsidewalls.
 22. The structure of claim 16 wherein the package substratecomprises a thin die and a low Z height.
 23. A structure comprising: asolder interconnect structure disposed onto a conductive pad of a bottompackage substrate; a molding compound disposed around the solderinterconnect structure, wherein there is an opening between the solderinterconnect structure and the molding; and a solder interconnectstructure disposed on a top package substrate, wherein the solderinterconnect structures of the bottom and the top package substratesform a low height solder joint structure.
 24. The structure of claim 23wherein a height of the solder interconnect structure disposed on thebottom package substrate is no greater than a height of the moldingcompound.
 25. The structure of claim 23 wherein a portion of the solderinterconnect structure disposed on the bottom package substrate is notin contact with a sidewall of the molding compound.
 26. The structure ofclaim 23 wherein the top and bottom package substrates comprise aportion of a POP.
 27. The structure of claim 23 wherein the solderinterconnect structure disposed on the bottom package substratecomprises a low height solder interconnect structure.
 28. The structureof claim 23 wherein the opening further comprises a side wall anglebetween the molding compound and the solder interconnect structuredisposed on the bottom package structure.
 29. The structure of claim 23wherein the low height solder joint structure comprises a ball on ballsolder joint.
 30. The structure of claim 23 further comprising whereinthe opening is capable of aligning the low height solder joint in thex-y direction.